1. Field of the Invention
The invention relates to a method for removing unburned carbon from fly ash, in particular a method for removing unburned carbon contained in fly ash discharged from coal fired power plants.
2. Description of the Related Art
Coal can be stably utilized as an energy source in the long term as the ratio of its proven reserves to annual production is more than 200 years. Therefore, the ratio of coal fired power generation to total power generation has been increasing year by year and the amount of coal ash generated (hereinafter referred to as “fly ash”) is expected to increase in the future.
In such circumstances, the large amount of fly ash needs to be efficiently utilized from the viewpoints of environmental conservation and the effective utilization of resources.
At present, fly ash is used as a cement admixture after having the unburned carbon removed from it. The quality of fly ash can be improved by increasing the removal rate of unburned carbon, which then enables the amount of fly ash used to be increased.
Therefore, the applicant has invented the method shown in FIG. 8, where slurry is generated by adding water to fly ash 61 in a mixing tank 62, a shearing force is applied to the slurry in a submerged stirrer 63 and then the unburned carbon in fly ash is efficiently removed in a floatation unit 67 (Refer to Patent document 1).    Patent document 1: Japan Patent No. 3613347
In the method described in Patent document 1, however, there has been concern that during the process of preparing slurry by adding water to fly ash, the required amount cannot be attained because fly ash adheres inside the feeding pipe and mixing tank, and moreover operation must be stopped because of the clogging of the pipe.
Specifically, when slurry 89 is attempted to be generated by stirring fly ash 87 and water 88 in the mixing tank 62 shown in FIG. 9, there has been problems that the fly ash 87 and the water 88 is insufficiently mixed because of the fly ash 87 adhering around the rotary shaft 81 of stirring blades 82 and that the fly ash 87 becomes wet because of water or water vapor in the mixing tank 52, resulting in clogging of a feeding pipe 84.